IBM Z: Define probe_stack expander

IBM Z: Define probe_stack expander

Probes emitted by the common code routines still use a store.  Define
the "probe_stack" pattern to use a compare instead.

gcc/ChangeLog:

2020-05-14  Andreas Krebbel  <krebbel@linux.ibm.com>

	* config/s390/s390.c (s390_emit_stack_probe): Call the probe_stack
	expander.
	* config/s390/s390.md ("@probe_stack2<mode>", "probe_stack"): New
	expanders.

gcc/testsuite/ChangeLog:

2020-05-14  Andreas Krebbel  <krebbel@linux.ibm.com>

	* gcc.target/s390/stack-clash-2.c: New test.